This invention relates to an electroless copper plating solution, and more particularly to a stabilized electroless copper plating solution for providing ductile copper.
Electroless copper plating is a chemical plating of copper on a surface of various substrates such as an insulating substrate, metal, ceramics and plastics by chemical reduction without an external electric current, and it has been used in various fields. See for example, U.S. Pat. Nos. 2,874,072 and 3,307,972, which disclose electroless copper plating solutions. The conventional solutions generally comprise cupric ions of a copper salt such as copper sulfate, a complexing agent for copper such as ethylenediaminetetraacetic acid in an amount sufficient to prevent precipitation of the copper ions in an alkaline medium, a reducing agent such as formaldehyde, and an alkali hydroxide such as sodium hydroxide. However, these conventional electroless copper plating solutions have various drawbacks such as poor stability of the solution and poor ductility and lack of brightness in appearance of the plated copper. These disadvantages are not serious when the electroless deposited copper is of the order of tenths of a micron in thickness and when a further layer of ductile and bright electrolytic copper is plated over the surface of the electroless deposited copper. However, when a comparatively thick layer e.g. having a thickness of over 30 microns is provided by an electroless coppering solution, for example in case of a printed circuit board, the above disadvantages become serious problem.
To overcome these problems, there have been proposed various methods in prior art. For example, U.S. Pat. No. 3,095,309 discloses use of a soluble inorganic cyanide as an additive for improving ductility of the deposited copper, and U.S. Pat. No. 3,804,638 discloses a polyalkylene oxide compound containing at least four alkylene oxide groups of two to four carbons per molecule in an amount sufficient to cause the resultant copper layer to be ductile. Further, U.S. Pat. No. 3,475,186 discloses addition of an organic silicon compound for improving strength, ductility and other properties of the deposited copper, and U.S. Pat. No. 3,615,732 discloses addition of a hydrogen inclusion retarding agent such as alkali and alkaline earth metal cyanides and nitrides, vanadium, molybdenum, niobium, bismuth, tungsten, rhodium, arsenic antimony, rare earths of the actinium series and rare earths of the lanthanum series; formaldehyde addition agent such as alkali metal sulfites, bisulfites and phosphite; and a salt of Group VIII metal such as iron, nickel and platinum for improving the bending or ductility properties of the deposited copper.